Galvanic cells, preferably nickel-cadmium cells, employing rolled or coiled electrode assemblies (jelly roll construction) are widely known in the art. In many of these cell structures the coiled electrode assembly is inserted into a composite housing which serves as the current conductive terminals for the cell. In such structures, the electrode of one polarity is electrically connected with a conductive housing portion, and the electrode of the opposite polarity is electrically connected with another conductive housing portion which is insulated relative to the electrically conductive first-mentioned housing portion. The electric contact between each electrode and its respective housing portion is generally formed by an elongated flexible electrically conductive connecting tab or element which is secured at one end to the electrode and at the other end is secured to the respective housing portion. When this type of cell is being assembled, the tab is first secured to the electrodes by welding or other securing means whereupon the opposite end of the tab is electrically secured to an appropriate conductive area of the housing. For example, the opposite end of the tab connected to the outermost wound electrode could be bent back upon the outermost separator layer of the assembly so that when the coiled electrode assembly is inserted into the housing, the tab will contact the inner surface of the housing (can portion) with sufficient pressure to provide a satisfactory electrical contact.
Cells employing coiled electrode assemblies can be produced using various electrochemical systems such as Leclanche, nickel-cadmium, alkaline MnO.sub.2, nickel-iron, nickel-zinc, nickel-hydrogen and the like. This invention however, can preferably be utilized in nickel-cadmium jelly roll cells. In the uncharged condition, the innermost electrode or positive electrode of a nickel-cadmium cell is nickelous hydroxide, the outermost electrode or negative electrode is cadmium hydroxide and the electrolyte is potassium hydroxide. In the charged condition, the positive electrode is nickelic hydroxide and the negative electrode is metallic cadmium.
The negative electrode of nickel-cadmium cells can be made by the rolling technique described in U.S. Pat. Nos. 3,310,437 and 3,432,351. As described in these patents, the carrier is usually a thin conductive strip such as a nickel strip or nickel clad (or plated) steel strip, lanced and stretched to give an open grid expanded metal. A mixture of electrochemically active material, binder, conductor and possibly other components are then coated on the carrier strip producing a negative electrode strip. The positive electrode may be made by the rolling technique as described in U.S. Pat Nos. 3,310,437 and 3,432,351 or may be impregnated nickel sinter as is well known and usually has an extended conductive tab, such as a thin nickel strip, welded to the longitudinal side of the electrode strip which when inserted in the can will be protruding through the open end of the can. The extended end of this conductive tab can then be suitably secured to the cover of the cell housing by conventional techniques such as welding or the like.
The negative electrode strip is generally the outermost wound electrode for nickel-cadmium cell systems and has a conductive tab secured to a selected area of the carrier at one end and to the cell housing at the opposite end. Conventionally, a conductive tab is welded to a selected area of the carrier by first removing the active electrode material about the area of the metal carrier to which the tab is to be attached. Generally, this removal process involves air blasting techniques. This removal process is followed by coining (pressing) the area of the carrier to be attached to the tab to reduce its thickness and flatten any burrs, and then the tab is spot welded to the selected area that has been coined. The electrode material could also be removed by scraping the material from the carrier or employing suction means to remove a portion of the electrode material from the selected area of the carrier. However, the employment of these methods to clean a selected area of the carrier has in some instances failed to remove all of the electrode material thus resulting in a poor welded connection to the conductive tab. Copending U.S. patent application Ser. No. 506,048 filed Apr. 9, 1990 discloses a method of cleaning a selected area of an electrode strip by using ultrasonic means provided by any suitable device, such as a prezo-electric oscillator, which can produce elastic waves of frequencies beyond the range of audibility generally in excess of 20,000 cycles per second. This application discloses that elastic waves of high frequency can be produced by quartz crystal oscillators designed for frequencies ranging up to 200 or 300 kilocycles per second. The disclosure made in this application is incorporated herein as if the entire disclosure were presented herein.
With the recent employment of conductive carriers made of fine diameter metal wire, such as nickel wire, to produce non-woven mat of low metal content (generally 5% to 10% by volume), a difficulty has been encountered in providing a good electronic securement of the carrier to a conductive tab. It has been observed that a conductive tab requires multiple welds to one side of the mat if a uniformly low resistance connection is to be made. In addition, the welding electrode that is placed against the mat generally becomes contaminated with small wire particles from the carrier during operation. When this occurs, the welding electrode has to be frequently cleaned or replaced resulting in inefficient production of the electrode strips.
It is an object of the present invention to provide an electrode strip with conductive tabs secured to both sides of a clean selected area of a perforated conductive carrier of the electrode strip.
It is another object of the present invention to provide an electrode strip made with a fine metal wire carrier and wherein a separate conductive tab is disposed onto each side of a selected area of the carrier and secured to the selected area of the carrier.
It is another object of the present invention to provide an electrode strip with an extended conductive tab that is cost efficient to produce when using a fine metal wire carrier.
It is another object of the present invention to provide a method for producing a good electrical connection between a conductive carrier of an electrode strip sandwiched between two conductive tabs so that said electrode strip would be suitable for use as an electrode of a coiled electrode assembly.
It is another object of the present invention to provide a method of electrically securing conductive tabs to a thin metal wire carrier of an electrode strip.
It is another object of the present invention to provide a method of cleaning a selected area of a perforated thin nickel wire carrier coated with an electrochemical active material and then welding a conductive tab onto each side of the cleaned selected area of the carrier.
Additional objects of the invention will become evident from the description and the drawings that follow.